1. Field of the Invention
The present invention relates to a system and method for enhanced monitoring of an etch process.
2. Related Art
In the prior art, a lamp emits a certain wavelength of light or electromagnetic radiation that is reflected from the surface of a substrate (e.g., a semiconductor wafer) to monitor an etch depth and to stop an etch process at a particular etch depth. For example, a mercury vapor lamp with a wavelength of 2537 angstroms may be used to monitor the etch depth of a semiconductor wafer. As semiconductor devices are manufactured with greater circuit density and smaller features, greater control of etch depth for shallow etches is required. Shallow etches refer to any etch that has an etch depth of approximately 2000 angstroms or less. To be able to accurately monitor a shallow etch process, a lamp or electromagnetic source may be selected to have a wavelength commensurate with (e.g., less than or equal to) the etch depth of approximately 2000 angstroms. However, lamps or light sources that have wavelengths equal to or less than approximately 2000 angstroms tend to be unstable, too expensive, unavailable, or otherwise not practical. Thus, a need exists for an etch monitoring system and method that supports monitoring of a shallow etch process with or without a lamp that has a wavelength of approximately 2000 angstroms or less.
Without an accurate monitoring method for an etch process, the etch process may form etches which are less than or greater than a desired etch depth. Deviation from the desired etch depth may impair the performance of semiconductor devices formed in the substrate. Accordingly, a need exists for accurate monitoring to support improved control of an etch process, such as a shallow etch process.
One aspect of the present invention regards a method for monitoring an etch process of a substrate that includes receiving a first signal having a first wavelength, deriving a second signal based on the first signal and combining the first signal with the second signal to produce a composite signal having a composite wavelength less than the first wavelength. The method further includes identifying inflection points of the composite signal and determining an etch rate of an etch process by evaluating the inflection points and elapsed time between the inflection points.
A second aspect of the present invention regards a system for monitoring an etch process of a substrate that includes a detector for receiving a first signal having a first wavelength, a signal source for deriving a second signal based on the first signal and a combiner arranged to combine the first signal with the second signal to produce a composite signal having a composite wavelength less than the first wavelength. An inflection-point detector identifies one or more inflection points of the composite signal and an etch rate calculator arranged to determine an etch rate of an etch process by evaluating the identified inflection points and elapsed time between the inflection points.
The primary advantage of the present invention is the ability to simulate a light source with a short wavelength without incurring the consequences associated with high frequency source. (ie. Cost, instablility, etc) This simulation allows for more frequent and accurate calculations of the instantaneous etch rate.
Further advantages, as well as details, of the present invention ensue from the following description of a preferred embodiment, making reference to the drawings.